In the field of low-voltage switching devices that are adapted to switch electric currents over voltages from 100 V up to 1000 V, electromagnetic trigger units, sometimes referred to as n-triggers, are implemented with an electromagnetic drive unit comprising a coil, a yoke and an armature.
An electromagnetic trigger unit is an unit that limits or breaks the current passing through an electromechanical switching device in the event of short-circuit. The trigger unit uses the force generated by the coil and yoke to push or pull the armature so that the current path is broken or limited. Electromagnetic trigger units are commonly used in circuit breakers and current limiters.
An electromagnetic trigger unit is usually adapted to break or limit the current if the rated current is exceeded. To this end, the coil in the electromagnetic drive unit must be able to generate a strong enough magnet field to pull or push the armature with enough force. For electromechanical switching devices that have small rated currents this may be a problem, since the magnet field is proportional to the current flowing through the coil.
For this reason, coils with a high number of windings need to be used in particular for an electromechanical switching device rated for a small current. Since such electromechanical switching devices tend to be rather small, the cross section of the winding must be small too to accommodate the high number of windings.
A winding having a small cross section is not very stable if large currents are carried through it, and so the copper wire of which the winding has been made tends to get permanently damaged in the case of short circuit.